Imaging systems are widely utilized to construct an image or model of a structure which is otherwise unobservable to the eye. Typically, imaging systems are designed to detect abnormalities, foreign objects or other structures which are embedded within a host medium and which alter or perturb the signal propagation properties of the host medium. For example, x-ray tomography and other medical imaging techniques are commonly used to create an image of a portion of the human body such that tumors or other inclusions can be detected. Similarly, imaging systems have been developed to detect deposits of oil or other minerals within the earth or to detect mines, such as mines buried underground or at sea.
By way of example, a variety of imaging systems have been developed to create an image of the human breast. These imaging systems are particularly important since breast cancer kills many women every year. For example, breast cancer is the leading cause of death for women ages 35 to 50 and the second leading cause of death for women over 50. The key to surviving breast cancer, however, is early detection and diagnosis. Currently, x-ray mammography is the most widely utilized technique for radiologically examining the human breast. Unfortunately, x-ray mammography exposes the patient to ionizing radiation which is a known cause of cancer. X-ray mammography also generally requires that the patient's breast be greatly compressed, such as to 4 centimeters, which can be quite painful. In addition, the images obtained by x-ray mammography techniques are not always of a sufficiently high quality to detect masses in the patient's breast, particularly for patients having radiodense breast tissue. The images produced by x-ray mammography techniques also do not clearly delineate between benign and malignant tumors. Thus, women who are found to have suspicious masses are generally required to undergo an invasive procedure, such as a biopsy, in which a portion of the mass is collected for analysis. At least in those instances in which the mass is found to be benign, the biopsy will generally have been unnecessary. As a result of the limitations of x-ray mammography techniques, a variety of other imaging techniques have been developed; particularly for the detection of breast cancer.
For example, magnetic resonance imaging, positron emission tomography, ultrasound imaging and thermography have been developed. Unfortunately, each of these imaging techniques suffers from a number of shortcomings. For example, while magnetic resonance imaging generally provides acceptable images, a magnetic resonance imaging machine is extremely expensive and is therefore not commonly utilized for breast cancer diagnosis.
Optical imaging techniques are now being developed as a potential alternative tomography technique. Since optical imaging utilizes non-ionizing radiation, the patient can be repeatedly or continuously exposed without harmful side effects. In addition, optical imaging techniques are non-invasive and are relatively economical relative to other imaging techniques, such as positron emission tomography or magnetic resonance imaging. Optical imaging techniques are also advantageous since the optical properties of the breast do not generally depend upon the patient's age and typically require only a gentle compression of the breast.
Similar to other known mammography techniques, optical imaging systems introduce light into a host medium, such as a patient's breast, and create an image of the host medium and abnormalities within the host medium based upon the interaction of the light with the host medium and the abnormalities. Due to differences between the optical properties of the host medium and the abnormalities, the abnormalities interact with the light in a different manner than the host medium. For example, the absorption coefficient and the scattering coefficient of an abnormality is typically substantially different than the absorption coefficient and the scattering coefficient of the host medium. Based upon the detected signals, an image of the host medium and abnormalities within the host medium can be created. Furthermore, optical characteristics of the host medium and, more importantly, abnormalities within the host medium can be determined. As such, optical imaging techniques offer the promise of permitting abnormalities to be characterized in a non-invasive manner. For example, optical imaging techniques may not only permit suspicious masses to be detected within a patient's breast, but may also permit benign and malignant tumors to be differentiated without requiring a biopsy or other invasive procedure.
Unfortunately, the strongly diffusive nature of light propagation in breast tissue significantly reduces the contrast and resolution of the optical images obtained by most optical imaging techniques. As such, optical imaging techniques have had difficulty consistently detecting and characterizing suspicious lesions, such as tumors, that are relatively small and/or deep within the breast.
With respect to these optical imaging techniques, a transillumination technique, also known as diaphanography or light scanning, was initially explored in which the patient's breast was illuminated with a continuous wave, broad beam light source. These transillumination techniques also employed a detector, such as a video camera, on the opposite side of the breast from the light source for detecting signals following propagation through the breast. Unfortunately, continuous wave transillumination techniques generally suffered from relatively low sensitivity and/or a relatively high number of false positive results.
As such, other optical imaging techniques have been developed that utilize laser light in order to provide images having increased resolution and sensitivity. These optical imaging techniques are practiced in both the time-domain and the frequency-domain. In the time-domain, the patient's breast is illuminated with a series of short pulses of light. By examining the manner in which the light pulses are altered during propagation through the patient's breast, an image of the patient's breast can be constructed. In the frequency-domain, however, the intensity of the light source is modulated at one or more frequencies, typically on the order of 108 Hz. Based upon the signals detected following propagation through the patient's breast, the phase shift and amplitude attenuation of the signals can be determined and a corresponding image of the patient's breast can be constructed. See, for example, the elliptic systems methodology described by U.S. Pat. No. 5,963,658 to Klibanov, et al., the contents of which are incorporated herein by reference. While these other optical imaging techniques are quite promising, particularly in conjunction with the early detection and characterization of breast cancer, further improvements to these optical imaging systems are desired in order to further improve the contrast and resolution of the optical images and the reliability with which physiological parameters that define the abnormality can be determined in order to accurately characterize the nature of an abnormality.